Emergency lighting system AC line voltage sensing

ABSTRACT

An emergency lighting system includes a high reactance transformer with a primary winding for connection to AC line voltage and a secondary winding. Circuit means are provided for monitoring the AC line voltage and includes means for coupling the transformer secondary winding to a non-linear load, a battery, during one AC half cycle and means for coupling the secondary winding to a linear load during an alternate AC half cycle to provide a DC voltage proportional to the AC line voltage.

This is a division of application Ser. No. 534,660, filed Dec. 19, 1974,now U.S. Pat. No. 3,921,005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high efficiency inverter circuitcapable of delivering sinusoidal voltage at high frequency, andparticularly to an emergency lighting system including a tuned inverterfor operating at least one gaseous discharge lamp.

2. Description of the Prior Art

Electric power failures due to inclement weather conditions andequipment breakdowns have been a plague for many years. Recently,widespread areas have suffered blackouts due to overloading of thegenerating or transmission equipment. A power failure, no matter whatmay be the cause, may very well jeopardize human life and thereforethere are many installations which require some type of emergencylighting system that will automatically come into operation upon theoccurrence of a power failure. The high efficiency of a fluorescent lampmakes it especially valuable for use in an emergency lighting system.

Many of the emergency lighting systems available on the market utilize arechargeable battery as the source of power for the system. Since thereis a finite limit on the length of time that a battery can power anillumination system, it is rather important that the system have a highefficiency. Presently available systems are generally of the type usinga transistor inverter. In a typical arrangement, a single lamp or groupof lamps is used for both the normal AC operation of the lighting systemand for the emergency system, using a battery as the power source forenergizing the transistor inverter when the AC line voltage fails. Aprinciple limitation of such inverter systems is relatively lowefficiency. This low efficiency requires the use of a larger and hencemore expensive battery to achieve acceptable operating time duringemergency conditions.

Prior art circuits for sensing AC line voltage in emergency lightingsystems are varied. One such system has a full wave responding inhibitvoltage developed from a separate winding than that used to charge thebattery, the inhibit winding being more tightly coupled to the primarythan is the battery charge winding. With such an arrangement, theinfluence of variable battery voltage is attenuated. Another systemappears to use a low reactance transformer with a dropping resistor toregulate the charging current to the battery, the resistor thuspreventing the battery from clamping the transformer winding andinfluencing the AC inhibit voltage.

It is desirable therefore to provide an emergency lighting systemcapable of maximizing the operating time on a given battery charge.

In accordance with the present invention, there is provided, in anemergency lighting system having a high reactance transformer with asecondary winding and a primary winding adapted for coupling to an ACenergy source, circuit means for monitoring the AC source voltage.Included are means for coupling the secondary winding of the transformerwith a non-linear load during one half cycle of the AC source voltageand means for coupling the transformer secondary winding with a linearload during an alternate half cycle to provide DC voltage proportionalto the AC source voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a basic schematic representation of the apparatus, includingthe high efficiency inverter, of the present invention;

FIG. 2 is a schematic representation of the preferred embodiment of theemergency lighting apparatus, incorporating the high efficiencyinverter, of the present invention; and

FIG. 3 is a detailed schematic representation of the preferredembodiment of the control circuit shown in block form in FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with the present invention and referring now to FIG. 1,there is shown a tuned inverter 10 for energizing a load such as gaseousdischarge lamp 12. A pair of transistors, Q_(A) and Q_(B) capable ofoperation in a low loss switching mode, are provided. An auxiliaryelectrical energy source, such as battery 14, provides the powernecessary to operate the inverter. Means enabling the transistors tooperate in the low loss switching mode are provided, such as inductorL1P, a buffer inductance, coupled serially with battery 14. A firsttransformer T1 serves to couple the inverter 10 with the lamp 12 and isresonated with capacitances C101 and C102 to set the operating frequencyof the inverter and to establish a sinusoidal output voltage. InductorL1P then is electrically connected with a center tap of a primarywinding P1 on transformer T1.

A control circuit 20 is provided for supplying base drive for switchingtransistors Q_(A) and Q_(B) at zero collector voltage: That is, when theinstantaneous voltage across capacitor C101 is zero. Thus, as theprimary voltage across transformer T1 varies at fundamental frequency,the voltage at point 22 and hence across conductor L1P varies at twicethe fundamental frequency. The current through L1P is DC with a secondharmonic component. This same current is alternately carried by the twotransistors Q_(A) and Q_(B). While the transistors are required toswitch collector current, they do so at essentially zero collectorvoltage with a resultant low power dissipation.

During some transient conditions, both transistors Q_(A) and Q_(B) mayswitch off thereby arresting current flow in inductor L1P. Voltageclamping means are provided in the form of inductor secondary windingL1S and diode D100 such that the ensuing voltage transient at point 22is limited, with the result that the stored energy is returned to thebattery. It should be noted that inductor L1S and Diode D100 carrycurrent only during transients and hence do not incur power loss duringnormal operation of the inverter.

Means providing timing information to the control circuit for effectingswitching of the respective transistors in step with a natural resonantfrequency of the inverter is provided and takes the form of an auxiliarywinding T1S2 magnetically coupled with the primary winding T1 of firsttransformer T1. Thus the control circuit tracks the resonant frequencyof first transformer T1 and ensures that transistor switching occurswhen the voltage across capacitor C101 is zero. Winding T1S2 is not usedas the source of energy for driving the transistors Q_(A) and Q_(B)because of its sinusoidal wave form.

Higher efficiency can be achieved in inverter 10 by making the basedrive of the transistors proportional to the collector current thereof.To this end there is included means providing a feedback current to thecontrol circuit to effect transistor base drive proportional totransistor collector current, which in the preferred embodiment takesthe form of a feedback transformer T2 having a feedback winding Dmagnetically coupled to the respective collectors of the transistorsQ_(A) and Q_(B) through a pair of windings A and B respectively. Thus,the power consumed by the control circuit 20 can be limited to thatrequired to start and control the oscillation of the inverter 10.

Referring now to FIG. 2, the basic inverter circuit of FIG. 1 has beenexpanded to provide an emergency lighting system circuit which willautomatically become operative upon the failure of the primary electricsource. Coupling of the lamps 12, 12' to an AC source through a ballast16 is shown in the conventional way for operation during normalconditions when the AC source voltage is above a predetermined value.The system includes means such as second transformer T3 for coupling toa first electrical energy source such as a typical 120 volt, 60 Hz ACsource. Means are provided for charging battery 14, this beingaccomplished through impedance limited transformer T3, thence throughdiode D101.

The operation of the system circuit of FIG. 2 will now be discussed withreference being made generally to the control circuit 20 shown in blockform. A thorough discussion will be given hereinafter of the operationof control circuit 20 by referencing to FIG. 3. It should be notedhowever at this point that control circuit 20 includes a first sensorfor sensing the voltage of the AC source and a second sensor for sensingthe voltage of the battery. Control circuit 20 also includes logic meanscombining the outputs of the first sensor and the second sensor toenable inverter 10 when the battery voltage is above a predeterminedlevel and the AC voltage is below a predetermined level and to disablethe inverter when the battery voltage is below a predetermined level orthe AC voltage is above a predetermined level.

Assuming now that the inverter is enabled to run, control circuit 20supplies a small base drive signal to one of the transistors Q_(A) andQ_(B). Assuming that the small base drive is applied to Q_(A), Q_(A)turns on and current starts to flow through L1, the center tap of theprimary P1 of transformer T1, thence through P1 and through the Awinding of feedback transformer T2, to transistor Q_(A) and thence backto the battery. The base drive originally supplied on transistor Q_(A)is augmented by a current flow from winding D of feedback transformer T2to the control circuit 20 to exit from pin 1 thereof thence to flowthrough the base of transistor Q_(A). This base drive is proportional tothe collector current of transistor Q_(A) and is designed to be adequateto keep the transistor in saturation.

At some volt-second product, feedback transformer T2 saturates sharply,suddenly reducing the output current of winding D thereof, therebyreducing the base drive to transistor Q_(A). A sudden rise incollector-emitter voltage on transistor Q_(A) sharply reduces the rateof current rise in this DC circuit. This change in collector currentwith respect to time reverses the polarity of the S2 winding oftransformer T1 and hence the polarity of the voltage on pins 3 and 4 ofthe control circuit 20. This reversal of polarity signals the controlcircuit to change the base drive from transistor Q_(A) to transistorQ_(B).

Control circuit 20 now applies a small amount of base drive through pin9 to the base of transistor Q_(B), and simultaneously connects the baseof Q_(A) to the emitter thereof to hasten the turn off process oftransistor Q_(A). Transistor Q_(B) starts to conduct as a result of thesmall base signal from the control circuit and current flows throughwinding B of feedback transformer T2 to induce a current in winding Dthereof and supplies this current to the control circuit 20. The controlcircuit now supplies this current as base drive out of pin 9 to the baseof Q_(B) ; thus the base drive of Q_(B) is proportional to the collectorcurrent thereof such that the transistor is kept in saturation.

Transformer T1 has an air gap and operates as a nearly linear inductor.When the voltage across winding P1 of transformer T1, and thereby thevoltage on winding S2 of that transformer, reaches zero, this event issignalled to the control circuit 20 through pins 3 and 4 thereof. Thecontrol circuit again switches the base drive circuitry to transistorQ_(A) from Q_(B) and connect the base of Q_(B) to the emitter thereof tohasten the switching off of transistor Q.sub. B. The circuit is thenready to go through the next half cycle with Q_(A) conducting.

If switching could be accomplished in absolute zero time, the abovedescribed circuit operation would be entirely correct. However, normallythe switching is accomplished in periods of less than one microsecondand the current flow from the battery 14 is essentially at a constantlevel with a small ripple content. This ripple content is determined bythe inductance of L1 which adds or subtracts from the battery voltage asapplied to the tap of the primary coil of transformer T1. It is thisinductor L1 which adjusts the voltage at point 22 in such a way that thetransistors may be switched at zero collector voltage. As long as thisinductance L1 has a value exceeding a critical value, the circuit willfunction as described. In the event that both transistors are in the offstate, the rate of current change in L1 forces the voltage thereacrossto a value where zener diode D104 starts conducting to limit the voltageapplied to the circuit. This clipping action rapidly reduces circuitefficiency and hence is an operational mode to be avoided. This clippingaction can occur momentarily during the starting process or when theinverter is turned off and under these conditions represents anacceptable design operating condition.

The load for the inverter 10 (including lamps 12 and 12') is connectedon a winding S1 of transformer T1. For fluorescent emergency lightingpurposes, the ballasting is done by capacitors C102 and C102' whichdetermine the load current through the lamps 12 and 12'. Thiscapacitance in conjunction with C101 and inductance of T1P1 determinethe operational frequency of the system. (The inductance of the P1winding and capacitance of C101 determine the oscillating frequency whenS1 is unloaded.) A double capacitive ballast system is used to reducethe voltage across a single unit and thus enhance the reliability of thecomplete system. The voltage output of the inverter circuit is highenough to instant start 40 watt rapid start lamps under fairly adverseconditions.

Circuit means are provided for monitoring the AC source voltage andinclude means for coupling the secondary winding S of high reactancetransformer T₃ with a non-linear load during one half cycle of the ACsource voltage, to supply charging current for battery 14. As statedabove, battery charging is accomplished from center tapped winding S of60 Hz transformer T₃. Half wave charging current is coupled to anon-linear load, the battery 14, through diode D101 and is limited inmagnitude by the reactance of the transformer. Because of thetransformer reactance, the sinusoidal voltage at the terminals ofwinding S is clamped at the battery voltage when diode D101 conducts. Onthe alternate half cycle, diode D103 conducts half wave current throughindicator lamp PL and the dual prong battery plug. Thus, the batterymust be plugged in and the 120 V AC power available to energize lamp PLindicating that the battery is charging. Using the alternate half cyclereduces the volt amp. rating of the transformer T₃. For monitoring theAC source voltage, means are provided for coupling secondary winding Sof transformer T₃ with a linear load during an alternate half cycle. Tothis end, during the half cycle alternate from that in which the batteryis charged, capacitor C104 is charged through diode D102. This DCmonitoring voltage is connected to the first sensor means at terminal 7of control circuit 20 through a linear load, resistor divider R104 andR105. The DC voltage at terminal 7 is proportional to the average valueof the 60 Hz supply voltage and is not influenced by the aforesaidclamping action of the battery. Thus, transformer T₃ serves a dualpurpose.

In the event that the battery 14 cannot accept charging current due to adefective cell or open connection, there exists the possibility that anabnormally high voltage from winding S on transformer T₃ would beapplied directly across pins 3 and 10 of the control circuit 20. Bytaking advantage of the current limiting characteristics of the windingS of transformer T₃, zener diode D104 conducts so as to clip the peaksof this voltage wave at a safe value, thus to protect the controlcircuit. This mean that zener diode D104 must be sized so as todissipate this expected energy.

Referring generally now to FIGS. 2 and 3, an explanation will be givenof the operation of control circuit 20 in conjunction with the inverterand the emergency lighting system.

A current set subcircuit 30 establishes the currents for the entireinverter control circuit. The master current for all current sources(Q1, Q2, Q3, Q11, Q12, Q22, Q23, Q25, Q30, Q31, Q32, Q41, Q42) is set at100 microamps, at room temperature, by resistor R101 coupled to the pin6 of the control circuit. The reference voltage for transistor Q7consists of diodes D1, D2 and D3 plus saturated transistor Q10. Thediode string is operated at approximately 100 microamps from currentsource Q2. Resistor R1 provides starting current to Q7 when terminal 5is above about 0.7 volts and the battery voltage is above approximately3 volts.

As stated above, the inverter must operate if the battery voltage isabove the DC inhibit threshold and the AC line voltage is below the ACinhibit threshold. Furthermore, if the battery voltage is below the DCinhibit threshold or the AC line voltage is above the AC inhibitthreshold, the inverter must not operate. To this end, the controlcircuit includes a first sensor in the form of an AC voltage inhibitsubcircuit 40 consisting of a differential amplifier Q13 and Q14, and acurrent minor load Q15 and Q16. Winding S of transformer T3 supplies ahalf-wave rectified signal over a diode D102 to filter capacitor C104and voltage divider network R104 and R105 to apply a signal to pin 7 ofthe control circuit. As this half-wave rectified voltage decreases withdecreasing line voltage, it finally reaches a point where the AC inhibitcircuit switches. This would be the inverter turn on point. Because ofthe nature of the half-wave rectified signal, a hysteresis is necessaryin the AC inhibit circuit operation. Thus, the inverter turn-off pointas controlled by the AC line will be higher than the inverter turn-onpoint. By adjusting the ratios of R104 and R105, either the inverterturn-on or turn-off point may be controlled over quite a wide range;however, both inverter turn-on and turn-off points may not be separatelycontrolled because of the relatively fixed value of the hysteresis builtin.

As stated, the AC voltage is rectified and filtered thence to flowthrough resistor divider R104 and R105 connected to input terminals 7.For high AC voltage, Q13 is cut off and Q17 conducts, establishing thereference at two diode drops (D4, D5+ VSAT (Q17). At low AC voltage, Q17is cut off and the reference voltage is raised regeneratively, by thevoltage drop across diode D6. The magnitude of hysteresis is one diodeD6 in series with D4 and D5. The swing between high and low voltage trippoints on terminal 7 is designed to consist of approximately 50% ripplevoltage and 50% change in AC voltage level. The relatively hightolerance to ripple voltage permits a smaller capacitor C104.

The DC battery voltage or charging transformer winding voltage isapplied between pins 3 and 10 of the control circuit. This same voltageis applied across voltage divider R102 and R103 to pin 5 of the samecontrol circuit. When the voltage at pin 5 drops below a valuedetermined by the construction of the control circuit and in particular,a second sensor in the form of low battery inhibit subcircuit 45, thecontrol circuit stops driving transistor Q_(A) and Q_(B) thus shuttingdown the inverter. This voltage is normally set at approximately onehalf of the nominal battery voltage but it may be adjusted by the ratioof R102 and R103. Hysteresis is implemented in the control circuit insubcircuit 45. Hysteresis provides clean on/off switching of theinverter. Furthermore after the inverter switches off, the voltage risescausing the inverter to operate again. This repeated flashing of thefluorescent lamp indicates the battery is discharged--a useful featurefor an emergency lighting system. The battery voltage is monitored byresistor divider R102-R103 connected to terminal 5. At low batteryvoltage, Q5 conducts, Q10 is cut off and Q18 conducts. The circuitregenerates and the magnitude of hysteresis is one diode (V_(BE), Q18)in series with D1, D2, D3. The presence of the starting resistor R1 isminimal at trip since the differential amplifier is essentiallybalanced.

In the event that the battery 14 cannot accept charging current due to adefective cell or open connection, the voltage across pins 3 and 10 ofthe control circuit will rise to a point which is the peak of the ACwave generated in winding S of the 60 Hz transformer T3. This voltagecould be excessive for the control circuit if certain design precautionsare not taken. When the voltage across pins 3 and 10 of the controlcircuit exceeds about 27 volts, an internal regulator in the controlcircuit shuts down the function of this control in such a way as tominimize the voltage stress on the various components of the circuit.

Transistor Q22 is subjected to nearly the entire voltage across pins 3and 10. In the event of an overvoltage fault, the zener diode string,D20, D21, D22, starts to conduct injecting current into transistor Q8thus shorting the bases of Q22, Q23, Q30 to their emitters throughtransistors Q18, Q10, Q9, Q8. This assures safe operation for Q22 up toa voltage across pins 3 and 10 equal to the collector to base breakdownvoltage of Q22 rather than the much lower collector to emitter breakdownvoltage.

In the overvoltage fault condition, the zener diodes D20, D21, D22conduct current and thus limit the voltage at the base of transistor Q21to approximately 27 volts. The emitter of Q21 will be at a voltage aboutone volt below its base or about 26 volts. It is this voltage that theremainder of the control circuit is subjected to under overvoltageconditions. The normal collector to emitter breakdown voltage for NPNtransistors fabricated by conventional integrated circuit technology isabout 30 volts. The addition of transistor Q21 and zener diodes D20,D21, D22 serve to increase the voltage withstand capability of thecontrol circuit from about 30 to about 50 volts, the remainder of thevoltage greater than 26 volts being applied across the collector toemitter of Q21. Thus, the reliability of the system is enhanced in thatelectrical stresses are better distributed in the control circuit. Inthe non-fault, normal battery voltage operating condition, the powerdissipation associated with the added protective components (Q21, D20,D21, D22, R14) is equal to the circuit current into pin 3 times the onevolt drop of R14 and Q21 base-emitter, thus preserving the highefficiency of the control circuit.

The voltage occurring during normal inverter operation across thetransistors Q_(A) and Q_(B) is double the DC, battery supply terminalvoltage. When these transistors are off, the voltage is reduced tosimply battery terminal voltage. If the voltage across terminals 3 and10 rises above nominal battery voltage, due to a defective cell or openconnection zener D104 starts to conduct and limits the voltage to lessthan 50 volts thereby protecting the control circuit. Current limitingwill be accomplished by means of the inherent impedance of winding S oftransformer T3. The voltage applied to the circuit is thus limited to asafe value under quite severe over-voltage conditions. In the event thatthe voltage continues to rise because of wrong voltage applied to theprimary of T3 and no battery, the most probable mode of failure of zenerdiode D104 is to short and this then crowbars the DC supply voltage tothe inverter.

As stated hereinbefore, logic means are provided in control circuit 20to combine the output of the first sensor, the AC voltage inhibitsubcircuit 40 and the second sensor, low battery inhibit subcircuit 45.This is in the form of a start-stop logic subcircuit 50 which includes acurrent source Q12 (nominal 100 microamps) which drives a current mirrordiode D6. The voltage drop across this diode is the reference for allcurrents in the remainder of the circuit. Transistors Q17 or Q18 shortout this diode in response to signals from the battery inhibitsubcircuit 45 or the AC voltage inhibit subcircuit 40.

Control circuit 20 also includes a zero crossing detector subcircuit 60which consists of a current mirror (Q19, Q20) driven from two 100microamp current sources (Q22, Q23). The AC signal is injected into theemitter of Q19 from a low impedance feedback winding S2 of transformerT1. This winding must carry 100 microamps when terminal 4 is positivewith respect to terminal 3. The detector is designed to switch with lessthan plus or minus five millivolts.

An amplifier subcircuit 70 in control circuit 20 converts a single endedinput to differential output. A differential amplifier (Q28, Q29) drivescurrent mirrors D7, Q49 and D8, Q39 approximately 180° out of phase. Thereference for the differential amplifier is two diode drops (Q26, Q21)below the positive bus. Transistor Q24 clamps the input to thedifferential amplifier at one diode drop below the reference andprevents current source Q23 from saturating. Current source Q30 drivescurrent sources Q25, Q26, Q31, Q32, Q41, Q42 which are referenced to thepositive supply for output drivers A and B.

A pair of output driver subcircuits 80 and 85 in control circuit 20provide the output drive for inverter transistors Q_(A) and Q_(B),respectively. The basic output driver subcircuit contains a 2 milliampcurrent source Q32, Q33, Q34, Q35, connected to output terminal 1through a Darlington switch, Q36, Q37. When transistor Q36 is cut off,the collector voltage on the Darlington rises, diverting the 2 milliampcurrent through D9, D10, Q38 and the base of Q40. Transistor Q40provides an active current sink on output terminal 1.

Once collector current starts in switching transistor Q_(A), feedbackcurrent from current transformer T2 flows through terminal 2, D11, Q37,terminal 1, Q_(A), D16 and terminal 8. This current is proportional tothe collector current in Q_(A) and is set by the turns ratio of T2(typically 20:1) to hold Q_(A) in saturation.

When the zero crossing detector subcircuit 60 switches, Q36 goes "off"and Q46 goes "on". The collector voltage of Darlington Q36, Q37 risesdiverting feedback current through D9, D10, Q38 and base emitter of Q40.The conduction of Q40 reduces the turn-off time of Q_(A). When thefeedback current driving Q40 ceases, Q40 remains in soft conduction dueto the aforesaid 2 ma base current and thereby provides a low resistanceacross the base emitter of Q_(A). The automatic reduction of basecurrent in Q40 conserves power and increases efficiency.

When Q46 is turned on, the 2 ma current from current source (Q43, Q43,Q45) is conducted through Q47 into switching transistor Q_(B) causing itto be in soft conduction. When the collector current in Q_(A) ceases,the feedback current reverses polarity, flowing through D15, Q47,terminal 9, Q_(B), D12 and terminal 2, causing Q_(B) to go intosaturation. Base drive is now proportional to the collector current inQ_(B).

When the zero crossing detector subcircuit 60 switches, Q36 goes "on"and Q46 goes "off". The turn-off of driver 85 and the turn-on of driver80 repeats in the manner as previously described.

If Q_(A) and Q_(B) are off simultaneously during switching, voltagetransients are produced. If Q_(A) and Q_(B) are on simultaneously,transient current will circulate between them. Over the normal range ofoperation, either condition may exist slightly.

Diodes D18 and D19, respectively, prevent excessive substrate currentflow when respective output terminals 1 and 9 are driven negative. Thiscondition exists if Q_(A) and Q_(B) are on simultaneously or under someinverter starting conditions.

Resistors R12 and R13, respectively, provide baseemitter resistance onrespective switching transistors Q_(A) and Q_(B) to minimize collectorleakage current when the inverter is not operating.

A free-wheel diode D17 provides a path for inductor L1 to discharge inthe event the battery is disconnected while the inverter is operating.

The control circuit 20 may be fabricated as a single monolithicintegrated circuit. In this form, the use of slaved current sources isparticularly practical. In the embodiment described, the currentconsumption and hence power dissipation in control circuit 20, isessentially independent of battery voltage over the operating range.Furthermore, the control circuit can be matched to different power levelinverters by scaling the currents in the control circuit. This isachieved by adjusting a single resistor R10l. Thus, by adjustment ofthis resistor and modification of transformer T3, the control circuitwould be applicable to a wide range of power levels while maintaininghigh efficiency.

The emergency lighting apparatus of FIG. 2, including the inverter ofFIG. 1 and the control circuit of FIG. 3, has been constructed and hasoperated satisfactorily with components having the following values:

    ______________________________________                                        transistors Q.sub.A, Q.sub.B                                                                 GE D42C10                                                      transformer T1 primary winding R1 600 uH -                                                   64 turns .0239" wire                                                          load winding S1 -                                                             1079 turns .0089" wire                                                        feedback winding S2 -                                                         7 turns .0089" wire                                            transformer T2 collector windings A & B -                                                    8 turns .0126"                                                                output winding D -                                                            160 turns .0071"                                               transformer T3 primary winding P - 1600 turns                                                secondary winding S - 420 turns                                               each side of center tap                                        inductor L1    120 turns .0359"                                               lamps 12, 12'  F40 T12 RS                                                     battery 14     18 VDC                                                         resistor R101  15 K ohms                                                       "   R102      56 K ohms                                                       "   R103      30 K ohms                                                       "   R104      18 K ohms                                                       "   R105      270 K ohms                                                     capacitor C101 0.22 uF.                                                        "   C102, 102'                                                                              5000 pF.                                                        "   C104      0.33 uF.                                                        "   C105      0.01 uF.                                                       diodes D101, 102, 103                                                                        1N4004                                                         zener diode D104                                                                             40 V ± 5%, 1/2 W                                            Control circuit 20:                                                            resistor R1   250 K ohms                                                       "   R2, R3   500 ohms                                                         "   R4, R8   2400 ohms                                                        "   R5, R9   240 ohms                                                         "   R6, R10  5 K ohms                                                         "   R7, R11  820 ohms                                                         "   R12, R13 2500 ohms                                                        "   R14      2 K ohms                                                       ______________________________________                                    

Control circuit 20 has been built and operated satisfactorily in bothdiscrete circuit form and as a monolithic .IC.

It should be apparent to those skilled in the art that the embodimentdescribed heretofore is considered to be the presently preferred form ofthis invention. In accordance with the Patent Statutes, changes may bemade in the disclosed apparatus and the manner in which it is usedwithout actually departing from the true spirit and scope of thisinvention.

What is claimed is:
 1. In an emergency lighting system having a highreactance transformer with a primary winding adapted for coupling to anAC energy source and a secondary winding, circuit means for monitoringthe AC source voltage comprising:means for coupling the secondarywinding of the transformer with a non-linear load during one half cycleof the AC source voltage; and means for coupling the transformersecondary winding with a linear load during an alternate half cycle ofthe AC source voltage to provide DC voltage proportional to the ACsource voltage.
 2. The circuit means of claim 1 wherein the transformersecondary winding is center-tapped.
 3. The circuit means of claim 2wherein the means for coupling with a non-linear load includes a diodeconnected serially with a battery between a first leg of the secondarywinding and the center tap for charging the battery.
 4. The circuitmeans of claim 2 wherein the means for coupling with a linear loadincludes a diode connected serially with a capacitance between a secondleg of the secondary winding and the center-tap, and a resistanceconnected in shunt with the capacitance, the DC voltage proportional tothe AC source voltage appearing across the resistance.